1. Field of the Invention
This invention relates to an optical scanning apparatus and an image forming apparatus using the same, and particularly is suitable for an apparatus such as a laser beam printer or a digital copier having, for example, the electrophotographic process adapted to reflect and deflect one or more beams emitted from light source means by a polygon mirror as a light deflector, and optically scan a surface to be scanned through an fθ lens system having the fθ characteristic to thereby record image information.
Particularly, the present invention relates to an optical scanning apparatus in which the shapes of a plurality of lenses constituting scanning optical means are appropriately set to thereby correct the curvature of image field in the main scanning direction and the sub scanning direction, the fθ characteristic and the fluctuation of the magnification in the sub scanning direction so that good images are always obtained, and an image forming apparatus using the same.
2. Related Background Art
Heretofore, in an optical scanning apparatus such as a laser beam printer, a beam light-modulated and emitted from light source means in conformity with an image signal is periodically deflected by a light deflector comprising, for example, a rotatable polygon mirror, and is converged into a spot shape on the surface of a photosensitive recording medium (photosensitive drum) by an fθ lens system having the fθ characteristic, and optically scans the surface of the recording medium to thereby effect image recording.
FIG. 18 of the accompanying drawings is a schematic view of the essential portions of an optical scanning optical system according to the prior art. In this figure, a divergent beam emitted from light source means 91 is made into a substantially parallel beam by a collimator lens 92, and this beam (the quantity of light) is shaped by an aperture stop 93 and enters a cylindrical lens 94 having refractive power only in the sub scanning direction. Of the beam having entered the cylindrical lens 94, that part in the main scanning section emerges intact and that part in the sub scanning section converges and is imaged as a substantially linear image near the deflecting surface 95a of a light deflector 95 comprising a rotatable polygon mirror.
The beam reflected and deflected by the deflecting surface 95a of the light deflector 95 is directed onto the surface of a photosensitive drum as a surface 97 to be scanned through an fθ lens system 96 having the fθ characteristic, and the light deflector 95 is rotated in the direction of arrow A to thereby optically scan the surface 97 of the photosensitive drum in the direction of arrow B (the main scanning direction) and effect the recording of image information.
To effect highly accurate recording of image information in an optical scanning apparatus of this kind, it is important that curvature of image field is well corrected over the entire surface to be scanned and the spot diameter is uniform, that an equal speed is kept when the surface of the photosensitive drum is light-scanned (fθ characteristic), that the lateral magnification in the sub scanning direction is uniformly corrected over the entire effective scanning area and the spot diameter in the sub scanning direction is uniform, and that in a multibeam scanning apparatus using light source means emitting a plurality of beams, the lateral magnification in the sub scanning direction is uniformly corrected over the entire effective scanning area and the pitch interval between scanning lines is made constant. Various optical scanning apparatuses or fθ lens systems satisfying such optical characteristics have heretofore been proposed.
For example, Japanese Patent Application Laid-Open No. 7-318796 discloses an fθ lens system comprising a combination of a glass toric lens and a plastic toric lens each having a cylindrical lens surface on the incidence surface side thereof and a toric surface on the emergence surface side thereof. In this publication, however, one surface is a cylindrical surface and therefore, there has been the problem that the degree of freedom is small with regard to the above-mentioned aberration correction and the above-mentioned aberration correction is difficult. So, in the present invention, as will be described later, all fθ lenses constituting an fθ lens system are made into toric lenses having toric surfaces on both surfaces thereof. Further, each of the aforementioned fθ lenses has both of its surfaces made into a non-arcuate main scanning sectional shape and has its radius of curvature in the sub scanning direction continuously varied, whereby the above-mentioned aberrations are corrected. Also, the above-mentioned publication does not bear the description of sub scanning magnification, and has not taken it into consideration to reduce the degree of sensitivity of focus fluctuation in the sub scanning direction and to uniformly correct sub scanning magnification in an effective scanning area on a surface to be scanned. The present invention takes these into consideration and can construct an optical scanning apparatus suited for the highly accurate recording of image information.
Also, in Embodiment 1 of Japanese Patent Application Laid-Open No. 7-318796, the power of a glass toric lens 22 on the scanned surface 14 side in the main scanning direction is greater than the power of a plastic toric lens 21 on the polygon mirror 12 side in the main scanning direction and therefore, a problem is left in achieving compactness. In Embodiment 2 of Japanese Patent Application Laid-Open No. 7-318796, both of the power of the plastic toric lens 21 in the sub scanning direction and the power of the glass toric lens 22 in the sub scanning direction are positive and therefore, there is left the problem that when the two lenses 21 and 22 are brought close to the polygon mirror 12 side, sub scanning magnification increases.
On the other hand, with the compactness and lower cost of laser beam printers, digital copiers, etc., similar conditions have also been required for image forming apparatuses.
What makes these requirements compatible is proposed, for example, in Japanese Patent Application Laid-Open No. 10-232346. In this publication, curvature of image field and distortion are corrected well and the influence of a change or the like in the spot diameter by image height is reduced.
However, to achieve further compactness of the optical scanning apparatus, it is necessary to shorten the focal length of the fθ lens system, widen the angle of field thereof and bring the fθ lens close to the polygon mirror which is deflecting means. All these are factors which make aberration correction difficult, and there has been the problem that when compactness has been made, the curvature of image field in a wide field angle area, the fθ characteristic, and the fluctuation of the magnification in the sub scanning direction are not corrected well.
Also, another problem arises with the widening of the angle of field. Heretofore, a beam emitted from light source means has been incident on the deflecting surface of the polygon mirror obliquely with respect to the optical axis of the fθ lens system, and at this time, the reflecting position at which the beam is reflected by the deflecting surface changes continuously and asymmetrically with respect to the center of scanning. This asymmetrical change in the reflecting position affects particularly the imaging position and it becomes difficult to obtain flat curvature of image field.
The above-mentioned asymmetrical change in the reflecting position is caused by making the beam from the light source means incident obliquely with respect to the optical axis of the fθ lens system and therefore, it can be eliminated by making the beam from the light source means incident from the direction of the optical axis of the fθ lens system, but the disposition is unreasonable and the beam must be made incident from the outside of the fθ lens system and therefore, the asymmetry given to the curvature of image field by the asymmetrical change in the reflecting position cannot be eliminated.
So, for example, Japanese Patent Application Laid-Open No. 4-60608 and Japanese Patent Application Laid-Open No. 9-265041 disclose various examples in which vertical asymmetry is introduced into the meridional shape of the fθ lens constituting the fθ lens system.
However, to achieve the compactness of the fθ lens system, curvature of image field, the fθ characteristic and the fluctuation of the magnification in the sub scanning direction must be corrected well even in a wide field angle area exceeding the angle of field ±47°, and these have not always been satisfactory.
Also, to make the optical scanning apparatus correspond to a multibeam, it has been necessary to make a beam emergent from a collimator lens into a substantially parallel beam in order to reduce the jitter in the main scanning direction.
Also, Japanese Patent Application Laid-Open No. 10-333069 discloses a construction in which in order to solve the problem arising in a multibeam scanning optical system that the relative interval between scanning lines is changed by the scanning position, the power distribution of a scanning lens and a correction lens in the sub scanning direction is designed such that the effect of correcting the curvature of image field in the sub scanning direction is obtained. However, the power of the correction lens nearest to the surface to be scanned in the main scanning direction is greatest and therefore, a problem is left in achieving compactness.
Also, in Japanese Patent Application Laid-Open No. 5-5852, in order to realize a bright fθ lens system, the fθ lens system is of two-unit two-lens construction, and prescribes the relations among the sub scanning magnification β, the composite focal length fs with respect to the sub scanning direction and the radii of curvature ry3 and ry4 of the third and fourth surfaces in the sub scanning direction. However, the power of the first lens near to a rotatable polygon mirror in the main scanning direction is smaller than the power of the second lens near to a surface to be scanned in the main scanning direction and therefore, a problem is left in achieving compactness.